This invention relates to the field of pagers and similar communication devices using binary digital address codes, and more particularly, to the provision of multiple function codes encoded within the address code.
Asynchronous digital detectors which require no bit or frame synchronization in order to detect the predetermined digital word or address are presently in use and have been disclosed in U.S. Pat. Nos. 3,801,956 and 3,855,576, both assigned to the same assignee as is the present application. The first of these patents discloses a system for asynchronously detecting one code word within a stream of data bits by cycling the sampled bits of the received data in parallel with the bits of the stored code address word, then counting the number of correlations. The second referenced patent discloses a system using asynchronous detection of the first address word to provide synchronizaton for detection of the second word. Thus only the first word need be one of a cyclic subset, and a large number of code addresses are made available. These same detection methods are also utilized in data terminals, where the address is followed by the transmission of a message or instruction, also in binary code. Data or message transmission in tone sequential pagers has been limited since two different tone signals are about the maximum that the user's ear and memory can distinguish. In the second patent referenced above, a technique is shown for encoding the address code to provide a maximum of four possible function codes. This technique involves transmitting one of the four possible combinations of the two words of the address and their complements, detecting the four combinations and providing parallel outputs for each detect circuit, but it has proven desirable to provide more than four function codes without adding another code plug.
It is also desirable to provide a pager with the above capability, which can be easily converted to use with two code plugs, and with four function codes associated with each separate address. Other useful features would be a status check for the individual elements of the visual display and for the power supply status.